This invention relates generally to zone plate photodetection, and more particularly, to such a detector that provides phase compensation.
Zone plate optics are well known in the art and not infrequently referred to as Fresnel zones after one of the early researchers in the field. While generally zone plate technology has been applied to imaging and radiation detection in various regions of the spectrum, including microwave and X-ray, they also have application with interferometers and spectrometers.
Concerning the radiation detection aspect of Fresnel zone plates, the most recent advances in the art involve the integration of zone plate optics with microcircuits and various electro-optic and photo-electric devices. These advances particularly in mineaturization have necessitated corresponding changes in the optical systems.
It is well known in the art that ring patterns inscribed on a plane have the ability to focus light. A zone plate consists of a series of coincentric rings in a plane, where the ring radii are given by the formula: ##EQU1## where K ring number from center
.lambda.=wavelength of light PA2 F=distance from zone plate to point of focus (focal length) PA2 .mu.=refractive index of medium through which light passes.
This equation (1) can be approximated by ##EQU2## for purposes of design. The rings given by (1) define zones which have an average distance d, from the focus of ##EQU3## Thus adjacent zones are separated from the focus by distances that differ by one half wavelength and hence light from these zones interferes destructively and light cancellation takes place.
However, if alternate zones are blocked, all light arrives at the focus "in-phase" and constructive interference is achieved with a resulting bright area or spot. A zone plate with alternate transparent and opaque zones is known in the art as an opaque-transparent zone plate.
Another zone plate structure is the phase reversal zone plate. This zone plate differs from the opaque-transparent plate by adding a dielectric to every other zone which has a thickness, ##EQU4## This added layer cuases a phase shift in the light of every other zone by radius, whereby all zones will transmit in phase.
Although the phase shift zone plate proves to be four times more efficient in focusing than the alternative opaque transparent zone plate, it is difficult to produce because of the thickness required by equation (4) is often large compared to the zone width in devices having more than a few rings. In these cases practical problems, such as etch undercutting of the dielectric during fabrication, limits the device resolution.
Accordingly, the invention described hereinafter involves a phase reversal zone plate detector that is simpler to produce and compatable with recent advances in the electronic detector field.